Tracking the GPS L2 signal plays a significant role in high-end GPS receivers and especially for civilian applications such as machine control and survey equipment. The GPS L2 signal, like the GPS L1 signal, contains three or more separate components: (1) a carrier signal (such as a 1227.6 MHz sinusoidal waveform); (2) a code signal; and (3) a data signal. The code signal is combined (modulated) with the carrier signal so that each GPS signal source, for example a satellite, can be distinguished from each other according to the particular code employed by that GPS source. The data signal contains the information which allows a GPS receiver to calculate its location, for example, the location of the transmitting GPS source.
The GPS L1 signal can include both a coarse/acquisition (“C/A-code”) code and a precision (“P-code”) code. However, since the early 1990s, the GPS satellites have been transmitting both the C/A-code and an encrypted form of the P-code called the “Y-code”. The Y-code is generated by encrypting the P-code with another code called the “W-code”. The W-code is secret and is not available for civilian use. Unlike the GPS L1 signal, the GPS L2 signal only includes one code, which since the early 1990s has been the Y-code. Because the W-code is unknown, the Y-code cannot be tracked by conventional procedures in a civilian GPS receiver. However, because the Y-code is formed from the P-code, in combination with the W-code, sufficient information about the Y-code can be ascertained by a GPS receiver to augment the GPS receiver's ability to track GPS signals. This type of tracking is termed “semi-codeless” because at least the contribution of the P-code to the Y-code signal is known even if the W-code contribution is unknown.
A number of semi-codeless tracking techniques have been developed, see for example, U.S. Pat. No. 4,972,431 to Keegan et al., U.S. Pat. No. 5,293,170 to Lorenz et al., U.S. Pat. No. 5,541,606 to Lennen, U.S. Pat. No. 5,621,416 to Lennen, U.S. Pat. No. 5,663,733 to Lennen, and U.S. Pat. No. 5,610,984 to Lennen. Each of these patents and any other documents referred to in this specification are hereby incorporated into this application by the respective reference.
However, none of these references apply known information about the timing pattern of the W-code.